1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof, and more specifically relates to an image forming apparatus and a control method thereof that performs image formation at lower tones than an input image.
2. Description of the Related Art
Some apparatuses, such as offset printing system, require a plate-making processing. On the other hand, some apparatuses such as inkjet system or electrophotographic system do not require that plate-making processing.
Printing that requires a plate-making process such as offset printing is suitable for printing various data in large quantity, such as printing of magazines or posters. However, it is known that plate-making processing costs much. In order to reduce the cost, before performing main printing, prepress is usually performed with a recording apparatus that does not require a plate-making processing, in order to check the pattern, color tone, document text, and so in an output subject. Consequently, a recording apparatus that does not require a plate-making process based on such as inkjet method or an electrophotographic method, is used as this prepress device.
When, for example, using an inkjet-type or electrophotographic-type recording apparatus as a prepress device, it may be necessary to reproduce an AM/FM screen or half-tone dots, in order to achieve the purpose. However, in an inkjet-type or electrophotographic-type recording apparatus, a decrease in image quality such as density unevenness or worsened granularity easily occurs, due to physical registration shift in main/sub-scanning and due to interference by an AM/FM screen or half-tone dots.
Consequently, a technique to improve the image quality to suppress density unevenness or the like, when attempting to reproduce an AM/FM screen or half-tone dots using the recording apparatus that does not require a prepress process, is proposed (for example, see Japanese Patent Laid-Open No. 2001-144959).
According to this technology, an inkjet-type recording apparatus, which have various inks with different density, is used. Noise data that imitates a screen or half-tone data is added to the image data, in order to determine the dot arrangement for relatively light ink. After the addition of the noise, half-tone processing with an error diffusion method or the like is performed on the image data. Thus, although accurate half-tone dot data is not reproduced, the resulting dot arrangement has high dispersiveness, as well as randomness. As a result, physical registration shift and interference by a screen or half-tone dot pattern can be prevented, and it is possible to reduce density unevenness and worsened granularity. Furthermore, interference can be suppressed by omitting processing to imitate half-tone dots in a high-density region.
However, according to the technique described in above Japanese Patent Laid-Open No. 2001-144959, there are problems as described below.
First, because merely noise data is added that imitates half-tone dots, it is unlikely that accurate half-tone dots are reproduced. Also, because using half-tone dots is avoided in a high-density region, reproducibility of half-tone dots extremely drops.
Further, although high dispersiveness can be obtained in the resulting dot arrangement, appropriate dot arrangements for each of multiple scans is not at all considered when using multi-pass (multi-scan) printing in which an image is formed by performing a plurality of scans. Accordingly, in multi-pass printing, dispersiveness of each scan could be low, and the ratio of dot coverage for a paper face could be changed. Thus, it is not possible to adequately suppress a deterioration of granularity or density unevenness, caused by the registration shift.